The present invention relates to an electronic apparatus, and more particularly to an electronic apparatus which, when put to an ambience test in which it is exposed to a flame, can prevent the flame from expanding.
Electronic apparatuses are configured of components excelling in incombustibility to ensure safety against and reduce damages by fires. This serves to eliminate the risk for electronic apparatuses to catch fire. However, the U.S. standard under GR-63-CORE, Section 4.2.2.2 of “NEBS Requirements: Physical Protection”, Telcordia Technologies, April 2002 requires that, even when an ignited burner (ignition source) is inserted into an electronic apparatus and methane gas is let in from outside to expose the printed circuit board to a flame, the fire should not propagate within the electronic apparatus. This condition of exposure to a flame is stated in “Equipment Assemblies—Fire Propagation Risk Assessment Criteria”, ANSI, T1. 319-2002, pp. 13-14, wherein the maximum volume of methane gas supplied to the burner, which is dependent on the height of the printed circuit board held perpendicularly in the electronic apparatus, and the methane gas supply profile are prescribed.
In a specific methane gas supply profile, as noted in FIG. 8A, first a burner ignited with methane gas that is let in at a flow rate of 1 l/min is inserted into a space between the printed circuit boards in the lower part of the electronic apparatus (time 0). From time 15 seconds until time 1 minute 25 seconds, the flow rate is increased linearly. The flow rate, which reaches its maximum (11.6 l) at time 1 minute 25 seconds, is linearly reduced to fall to 0 l/min by time 4 minutes 30 seconds. Then, a minimum flow rate of 1 l/min is maintained, and the supply of methane gas is cut off at time 4 minutes 30 seconds. Thus, the printed circuit boards and electronic components mounted on the printed circuit boards on the two sides of the burner are exposed to a flame for 4 minutes 30 seconds. While the position of inserting the burner is prescribed to be underneath the mounted part of the printed circuit boards, its slot position can be determined by the examiner at his or her own discretion.
The specification of the U.S. Pat. No. 6,927,977 discloses an electronic apparatus whose metallic baffle disposed in the upper part of the apparatus is cooled by a blower to satisfy the aforementioned requirement.
An electronic apparatus is usually subjected to forced air cooling by the use of a cooling fan, whose speed is controlled. Cooling fan speed control is a technique by which the fan speed is normally kept slow to suppress noise and, when a temperature rise is detected by a sensor, is raised to prevent the temperature from rising. The cooling fan is intended to maintain the temperature within the apparatus from rising above a certain level, and is usually controlled on the basis of comparison of the absolute temperature level detected by the sensor arranged in the upper part of the case in accordance with a set threshold.
JP-A-250489/1997 discloses an invention according to which, in order to cool and ensure stable operation of an electronic apparatus by revolving a cooling fan at an appropriate speed, plural temperature sensors are disposed in plural positions in a box housing the electronic apparatus, and the frequency of revolutions of the cooling fan of the electronic apparatus is controlled according to temperature signals obtained from those temperature sensors. However, since the frequency of revolutions of the fan is determined according to the absolute temperature level according to this technique, application of this technique to detection of a flame would involve a problem that, the detection of flame generation will be delayed if the ambient temperature does not reach a high level in a short period of time because the flame arises when the ambient temperature of the temperature sensors is low or for any other reason.
JP-A-63237/1996 discloses a reliable cooling system for electronic devices which utilizes light emitting elements to perform cooling control to accurately address abnormal high temperature of components whose temperature is measured. This system, however, requires arrangement of a sensor in every position where temperature monitoring is desired, but hardly allows monitoring of flame occurrence with a relatively small number of temperature sensors.
JP-A-065267/1995 discloses a fire alarm which, when a predetermined temperature has been reached or a temperature rise per unit time has reached a predetermined level (25° C. per minute in an embodiment of the invention), determines the temperature change as indicating a fire accident.